Almost a Time Machine
Mechanical TV is almost like stepping into a time machine: A time machine that
we've built with our own hands and minds. Mechanical TV from our own camera
adds a new dimension to the experience. Now, we can take and share pictures, test
different circuits and learn more with every improvement.
In the diagram above, Block 1 is our pickup device. For Block 1, many experimenters use
a small solar cell or photodiode. Solar cells can pick up a fairly
broad spectrum of light. If you use a solar cell, stick with a small one. Larger
cells respond too slowly for this project. Photodiodes respond very quickly to
light changes. Unfortunately, photodiodes require extra amplification.
Easy to use. My personal preference
for Block 1 is a phototransistor. It's not as quick as a photodiode,
but it offers some gain. Also, a phototransistor is more durable and
easier to work with than a solar cell. See the phototransistor circuits
nearby. You can use many types of phototransistors in these circuits.
Two that I've used recently are...
- Texas Instruments TIL414 from RadioShack
- Ledtech LT9593-91-0125 from All Electronics
High-gain phototransistor circuit
for mechanical video pickup
Requirement: Clear case. Make sure that you use a
phototransistor with a clear, plastic envelope or
case. Types with black cases won't work for this project.
A black case blocks visible light. Some infrared phototransistors have
these black (or dark color) cases. Other devices have a clear case, and
respond to both infrared and visible light. These dual-purpose types
are fine. Radio Shack,
and All Electronics
carry suitable phototransistors.
Install the phototransistor in a rectangular project box. Bud® makes suitable
boxes. You'll find such boxes at Radio Shack, Mouser, and other vendors. The box must have plenty of
room for your pickup and a preamp. Allow a couple of extra inches between the end of the box and the
pickup cell. Of course, the box must not be too large to fit inside your scanner. The light pickup will
face a window that you'll cut in one short side of the box. This side becomes the front of the box. In a
moment, I'll give more details about the window. You'll mount your PC board in the back of the box.
Circuit with faster response
speed, but voltage gain of 0.9
The window should be about the size of our video picture frame, or slightly larger. Mount the
phototransistor about two inches behind the window. The window and phototransistor must face the
Nipkow disc apertures.
Preamplifier blocks 2 and 3 tend to be difficult for beginners to design and
build. If you're determined, curious or persistent, forge ahead anyway.
Beginners often surprise themselves with their success.
How do we design a phototransistor preamplifier? We start by examining
circuits. Maybe we can use or modify something that already exists. There are
lots of circuits, and many are easy to build. Below is one idea that might
Low-level preamp. Our low-level preamplifier
Q2 is an NPN, common-emitter stage. Our phototransistor is Q1.
The Q1 circuit is an emitter-follower stage. We direct couple this stage
to Q2. Output signal Vo comes off Q2 between collector and ground.
Transistor Q2 is a general-purpose, silicon, small-signal type.
For example, 2N2222, 2N3904 or 2N4401.
(Hfe: 200 or above. Germanium types won't work in this circuit.)
Low-level preamplifier improves
Limits. The signal is still a high-impedance one. Yet we have a much
stronger signal than we started with. At 1 kHz, Q2's unloaded
gain is 179. Unfortunately, this is a pie-in-the-sky
gain figure. When we connect Q2 to anything meaningful, our
gain crashes back toward earth.
Capacitors. You'll notice the large decoupling capacitor. We need Paul
Bunyonesque capacitors so that we can reproduce frequencies
down to nearly DC. These frequencies account for crucial
large picture details.
Need: More Gain! Our new circuit is helpful for experiments. Yet we probably still
need more voltage gain. I suspect that our solution requires two or three more transistors.
On the next page, I explain how to add transistor stages. Believe it or not, there's a very
easy way to do that!
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